Many therapeutic devices use electrodes to measure electrical potentials at selected locations in a human body. Electrodes have been used to measure electric signals relating to chemical concentrations, nerve activity, respiration rate, or cardiac rhythms, as examples. Cardiac pacemakers, for example, stimulate the heart with electrical impulses to induce a heartbeat. Pacemakers may also sense the condition of the heart so that stimuli may be applied in an appropriate manner. Electrodes on a lead of a cardiac pacemaker may be used to measure features of the intracardiac electrogram, lead or tissue impedance, pH of the blood and other parameters. In addition to the desired, information carrying signals, the electrode may detect unwanted signals such as remote or far field signals, noise, muscle activity potentials and so on. The sense electrode or another electrode may be used to stimulate a physiological mechanism by an appropriate signal; for example, stimulating the heart to induce a heartbeat. Such a stimulation can result in a residual polarization voltage which masks the desired signal.
Accurate signal detection through an implanted electrode continues to present difficulties for the designer of an implanted therapeutic device. In some applications, it may be desired to detect one or more input signals and to apply one or more stimulating or output signals. At the same time, it may be inadvisable to employ a dedicated electrode for each operation. The desired size of a lead may limit the number of conducting wires which can practically be connected to separate electrodes or the number of electrodes themselves may be limited. Moreover, interelectrode cross-talk between separate electrodes may limit the usefulness of separate electrodes.
To solve these problems, time multiplexing of an electrode has heretofore been employed. The same electrode used to output a stimulating pulse during a first interval may be employed to sense cardiac signals during another interval and noise or some other parameter during yet another interval. Bandpass filtering has been used to discriminate between desirable signals and noise. These methods, however, are not effective in all cases. Unwanted signals may mask desired signal in certain applications. In other applications, a signal of interest may mask another signal of interest, requiring the rejection of the first signal in order detect the second.
With the foregoing in mind, it is an object of my invention, therefore, to provide an apparatus and method for an implanted therapeutic device having an electrode which can be used for multiple functions.
It is also an object of my invention to provide an apparatus and electrode which can be used for both input and output functions.
Another object of my invention is to provide an implanted apparatus which can extract information from a background signal during a selected interval despite the presence of a primary signal which tends, during another time interval, to influence the electrode potential to destroy the electrode's ability to sense the background signal.